Melanocortins are a family of regulatory peptides which are formed by post-translational processing of pro-hormone pro-opiomelanocortin (POMC; 131 amino acids in length). POMC is processed into three classes of hormones; the melanocortins, adrenocorticotropin hormone, and various endorphins (e.g. lipotropin) (Cone et al., Recent Prog. Horm. Res., 51:287-317, (1996); Cone et al., Ann. N.Y. Acad. Sci., 31:342-363, (1993)).
Five melanocortin receptors (MC-R) have been characterized to date. These include melanocyte-specific receptor (MC1-R), corticoadrenal-specific ACTH receptor (MC2-R), melacortin-3 (MC3-R), melanocortin-4 (MC4-R) and melanocortin-5 receptor (MC5-R). All of the melanocortin receptors respond to the peptide hormone class of melanocyte stimulating hormones (MSH) (Cone et al., Ann. N.Y. Acad. Sci., 680:342-363 (1993); Cone et al., Recent Prog. Horm. Res., 51:287-318 (1996)).
There has been great interest in melanocortin (MC-R) receptors as targets for the design of novel therapeutics to treat disorders of body weight such as obesity and cachexia. One of the receptors, MC4-R, is a 332 amino acid transmembrane protein expressed in brain as well as placental and gut tissues (Cone et al., Ann. N.Y. Acad. Sci., 680:342-363 (1993); Cone et al., Recent Prog. Horm. Res., 51:287-318 (1996)). Recent pharmacological confirmation has established that central MC4-R receptors are the prime mediators of the anorexic and orexigenic effects reported for melanocortin agonists and antagonists, respectively (Giraudo et al., Brain Res., 809:302-306 (1998); Farooqi et al., NE J Med., 348:1085-1095 (2003); MacNeil et al., Eu. J. Pharm., 44:141-157 (2002); MacNeil et al., Eu. J. Pharm., 450:93-109 (2002); Kask et al., NeuroReport, 10:707-711 (1999); Chen et al., Transgenic Res., 9:145-54, (2000); Marsh et al., Nat Genet., 21:119-22, (1999); Balthasar et al., Cell, 123:493-505 (2005)).
Complications of body weight disorders commonly include an inability to produce and utilize insulin, often resulting in faulty glucose regulation. The consequence of failure to properly control glucose metabolism affects many aspects of overall health including energy metabolism, neuropathy and heart disease. Current progress with receptor-selective melanocortin receptor ligands evidences the therapeutic potential of melanocortin receptor activation, particularly MC4-R, in the treatment of glucose regulation, including insulin metabolism.